There is a tremendous demand for sheets or other substrates having coated thereon thin layers or "films" of liquids, in particular, polymeric liquids such as pressure-sensitive adhesives (PSAs). Such PSA liquids fall into at least three categories, including emulsions, hot melts, and solvent-based solutions; however, there are numerous types of PSAs within these and other categories exhibiting a wide variety of fluid characteristics. There are also numerous other kinds of liquids which require coating onto some type of substrate.
Typically, such a substrate with the thin film coating thereon is formed into rolled materials, which then undergo a "converting" process wherein they may be printed, die cut, and otherwise formed into a wide variety of end products, including labels, identification systems, tapes, etc. These rolled, coated materials often exhibit a sandwich construction, meaning that the substrate is coated with multiple layers of liquid PSA adhesives or other liquids which then receive a top sheet comprising some type of facestock. There is almost an endless variety of such multilayer products made up of numerous different kinds of backing sheets, coatings, and facestocks.
At present, in the production of such multilayer products, each layer is typically coated individually in a single pass through a coating device. The coating may be applied to any type of substrate, including a release liner or even to the facestock. The coating is then typically oven dried or solidified by cooling in the case of hot melt PSAs. If additional layers of coatings are to be applied thereon, the rolled material, having previous coating layers applied thereto, undergoes another coating operation. Ultimately, it is common for a backing and a facestock, each having any number of layers applied thereto, to be laminated together to form the final multilayer product. A number of coating techniques may be utilized; however, interference coating or proximity coating is commonly used for the single-layer coating of the type described. In either case, the liquid to be coated in a single layer on the substrate is fed past an elongated slot formed in a die (thus, this technique is also sometimes referred to as "slot coating"). The slot is positioned at approximately a right angle to the direction of travel of the rolled substrate, which is usually referred to as a "web." The die is stationary, but the head of the die, comprising two "lips" which define the opening of the slot, are placed adjacent to the web. The web travels around a back-up roll as it passes in front of the lips. The slot formed by the lips and the web have substantially equal widths, such that the entire cross web width of the web is coated in one pass by the fluid as it flows out of the die and onto the moving web.
If properly designed and adjusted, the die will distribute the liquid evenly and uniformly across the web in a thin layer. Typically, the die can be adjusted radially to move toward or away from the web, thus determining the gap between the lips and the web, also referred to as the "coating gap." In addition, the angle of the lip surfaces with respect to the web, or "angle of attack," can also be adjusted. For a given coating thickness, the flow parameters of the liquid can be determined, including the flow rate. Once these parameters are determined and the die is "set" in the coating machine, usually only the coating gap and angle of attack are adjusted during operation. However, because of the extremely thin layers being coated, any such adjustments usually inject a certain degree of imprecision into the process.
For example, it is common for such single-layer coatings to be in the range of 2-50 microns. Moreover, the difficulty in accurately coating such layers is increased by their relatively high viscosity, usually in the range of 50-50,000 millipascal-seconds (mPa-sec). In addition, the pressures and shear rates experienced during coating often will vary by several orders of magnitude. For example, some types of PSA liquids experience pressures in the range of 900 psi. The die must be able to coat liquids having these parameters at relatively high production rates, e.g., web speeds in the range of 50-350 meters per minute or higher.
There are also physical limitations on the accuracy of the die itself. For example, it is very difficult to hold extremely small tolerances on the lip geometries of the die, especially over the width of the slot which may vary between a few and a hundred or more inches. Thus, in order to achieve as much precision as possible, in the case of interference coating the lips of the die are actually pressed forward into the web which is supported by a back-up roll typically constructed from a hard rubber material, which in turn deforms in response to the forward pressure of the die. The downstream lip and most of the upstream lip do not contact the web because they hydroplane on a thin layer of liquid, although in some cases a portion of the upstream lip can contact the web. Thus, such deformation compensates for any imprecision in the configuration of the die lips. On the other hand, this technique has the disadvantage of increasing the rate of wear of the die lips (especially the upstream lip), further injecting inaccuracies into the process. Moreover, under these circumstances, any imperfections in the roll (e.g. eccentricities or "roll runout") will be magnified. Another disadvantage of interference coating is that the passage of a splice in the web may be difficult.
In another type of coating, proximity coating, the lips of the die are set back a precise distance away from the web. The back-up roll is typically constructed from a stainless steel material which allows for precision in the circumferential shape of the roll. Thus, unlike interference coating, the back-up roll in proximity coating is less likely to exhibit eccentricities (also referred to as "roll run-out") as it rotates.
To further achieve precise single-layer coating, a number of techniques have been developed. For example, it is well known that the configuration of the lips can be adjusted with respect to the web in order to improve coating accuracy and uniformity. Also, it is well known to angle or cant the downstream lip of the die so that it is somewhat convergent with respect to the web. This has the advantage of providing a smooth surface for the coating and avoids "ribbing" and other defects in the coating. This lip convergence is typically accomplished by adjusting the angle of attack of the die so that the lips are angled to face the oncoming web (defined herein as negative degrees of angle of attack).
However, adjustments in the angle of attack of the die affect the fluid mechanics of the overall "bead" of liquid. The bead is defined as that portion of the liquid captured between the die lips and the web, along the two longitudinal sides, and between the two ends of the bead defined as the upstream meniscus and the downstream meniscus or film-forming region. Thus, if the convergence is too large, the flow sees a large pressure gradient which has a tendency to force the liquid upstream. If the bead advances in the upstream direction, it is likely to explode, since the pressure gradient varies quadratically in this region. This results in "upstream leakage" of the liquid, obviously resulting in poor coating performance. Therefore, another single-layer coating technique is to position the upstream lip so as to increase the pressure drop along this die lip. This has the effect of ensuring that the bead remains under the lips or is "sealed."
Another disadvantage of such larger pressure gradients is the resulting shear rate experienced by the liquid. In single layer coating where viscosity is determined only by the properties of one liquid, the negative side effects of such high shear rate are limited to poor film quality whenever the high shear stresses redistribute the film in the cross-web direction, or when they cause material breakdown in shear sensitive liquids. Additionally, for multilayer coating, where viscosity may vary due to the existence of multiple liquids, although not completely understood, it is observed that this high shear rate (or even a lower shear rate experienced over a given period of time) causes the fluid to vary from a stable, two-dimensional flow to take on a three-dimensional flow profile. In other words, the flow, in the face of shear stresses, attempts to rearrange itself into a three-dimensional pattern in order to reduce the resistance to flow. As a result of this three-dimensional flow, the liquid undergoes a certain amount of convective mixing in between the layers.
There are other sources of imprecision in single-layer coating. For example, it may be difficult to correctly control the viscosity of the liquid or the velocity of the web. The web itself may be a relatively uneven or irregular surface, thus increasing the difficulty in applying a uniform coating thickness thereto. Foreign particles or other materials may be deposited onto the web or entrained into the liquid. Moreover, even slight variations in ambient pressure can affect coating accuracy. Any one of these events can result in a "perturbation" or variation from steady-state coating.
Notwithstanding the foregoing difficulties, good results can usually be obtained with present single-layer coating techniques. The process can be quite forgiving. That is, perturbations or other instabilities often do not have a substantial effect on the performance of the end product. In addition, if the flow is stable, the effect of a perturbation is likely to dampen out very quickly, thus minimizing the severity of the defect.
However, there is an ever-present need to reduce production costs and to develop higher quality products. In the single-layer coating process described above, a number of coating, drying, and laminating steps must occur to produce a final multilayer product. Thus, the costs of machinery and labor are relatively high. Also, it has been found that the mechanical and rheological properties of certain multilayer products may be different depending on whether the layers are coated individually or simultaneously. That is, if two wet layers are applied simultaneously to a substrate, it has been found that the end multilayer product may have improved convertibility and performance. However, in order to coat two or more layers simultaneously, the die must have two or more slots instead of one. Thus, in addition to an upstream lip and a downstream lip (which are used for single-layer coating), a multilayer die must also have intermediate or "middle" lips in order to define the appropriate number of slots or feed gaps.
Such "dual" dies, however, have not yielded successful multilayer coatings. This is because the principles of single-layer coating do not translate well into multilayer coating. The fluid mechanics of two or more wet layers simultaneously applied to each other are very different than those experienced in a single layer. On the other hand, in certain industries, such as the photographic film industry, multilayer coating has been successfully utilized in a number of coating techniques, including slide coating, combination die/slide coating, or straight die coating. However, the liquid requirements of that industry are quite different from the PSA industry where highly viscous liquids are prevalent.
Furthermore, in the PSA industry, high performance products for specific applications are frequently required. Thus, the adhesives used in these applications may be specially designed individually and strategically combined as a pair into the multilayer product. This situation may present a severe challenge to the coating expert since the product is engineered for its performance characteristics or the properties of the adhesive, and not for its coatability.
Thus, there is a need in the prior art for a method of multilayer die coating utilizing a wide variety of liquids wherein the coatability of certain combinations of viscous adhesives can be adjusted or otherwise improved while maintaining the important properties of the adhesives.